Infection Mechanisms and Host Immune Responses

Infection

The replication of organisms within a host's tissue, representing an invasion by harmful microorganisms or parasites.

Inapparent or subclinical infection

An immune response without obvious clinical disease.

Carrier

An individual harboring organisms that may be multiplying, without showing clinical symptoms.

Chain of infection

Consists of the etiologic agent, the method of transmission, and the host.

Etiologic agent's pathogenicity

Includes virulence (disease severity) and invasiveness (ability to invade tissue).

Transmission modes

Include contact, common vehicle, air, and vectors.

Opportunistic infections

Infections that can occur in immunocompromised patients.

Bacterial infections

Often begin with adherence to a specific epithelial surface.

Tissue injury by bacteria

Caused via exotoxins, endotoxins, non-specific immunity, and specific humoral and cell-mediated immunity.

Viral infection

Involves attachment to the cell surface, penetration, and uncoating.

Viral pathogenesis

Includes implantation at the entry portal, local replication, and spread to target organs via viremic or neural routes.

Incubation period

The time between exposure to the virus and the onset of the disease.

Fungal disease mechanisms

Can cause disease through cellular injury, toxic metabolites, replication, and immune response.

Fungal infection portals

Fungi infect the body through various portals of entry, but rarely cause disease in healthy hosts.

Severity of fungal disease

Depends on inoculum size, tissue destruction, fungal multiplication ability, and host's immunologic status.

Parasites

Rely on hosts for survival, causing harm by consuming resources and releasing toxins.

Parasite transmission strategies

Include penetration and ingestion.

Protozoal protective mechanisms

Include antigenic masking, blocking, intracellular location, antigenic variation, and immunosuppression.

Helminth transmission

Transmitted to humans via ingestion of eggs/larvae, skin penetration, insect vectors, or consumption of intermediate hosts.

Factors influencing susceptibility to helminths

Include hygiene, climate, food preparation, and exposure to insect vectors.

Helminthic pathophysiology

Involves direct damage from worm activity (blockage, pressure, tissue alteration) and indirect damage from host response (hypersensitivity, inflammation).

Innate immunity system

Attacks all antigens equally.

Physical barriers

Include intact skin, ciliary action, coughing, sneezing, flushing action of tears, saliva, and urine.

Protective cells of the body

Include phagocytes, natural killer cells, and polymorphonuclear neutrophils (PMNs).

B cells

Produce antibodies and manage humoral immunity.

T cells

Are non-antibody-producing and form the basis of cell-mediated immunity.

Antibody-mediated immunity

Involves antibodies (immunoglobulins) that inactivate antigens through complement fixation, neutralization, agglutination, and precipitation.

Cell-mediated immunity

Involves cytotoxic T cells (CD8+) releasing lymphotoxins, helper T cells (CD4+) directing the immune response, and suppressor T cells slowing down the immune response.

Vaccination

Involves administering antigenic material to produce immune protection against a disease.

Active immunization

Involves giving antigens to stimulate antibody production.

Passive immunization

Administers actual antibodies.

Inactivated vaccines

Use killed virus particles requiring booster shots.

Attenuated vaccines

Use live virus particles with low virulence but are unsuitable for immunocompromised individuals.

Subunit vaccines

Present an antigen to the immune system without introducing viral particles.

Bacterial vaccines

Include those against anthrax, cholera, diphtheria, pertussis, tetanus, Hib, and meningococcus.

Viral vaccines

Include those against adenovirus, tick-borne encephalitis, Japanese encephalitis, flu, hepatitis A and B.

Other vaccines

Target malaria, trypanosomiasis, schistosomiasis and hookworm.